1. Field of the Invention
The present invention relates generally to the field of fluid sampling, and more specifically, to a mobile sampling system for taking samples from a natural gas well that is at low pressure (less than 5 psig) or under vacuum and analyzing those samples in the field.
2. Description of the Related Art
In most cases, after years of production, the amount of natural gas available for production from a given well will decrease. The rate and quantity of the decrease will vary from well to well, but for many wells, it eventually becomes necessary to lower the pressure (to less than 5 psig) or place the well under vacuum in order to cause the natural gas to flow out of the well. In order to lower the pressure or place the well under vacuum, compressors, pumps and/or blowers are added to the pipeline system downstream of the wellhead electronic flow meter. Typically, the well operator can take a sample from the natural gas stream and analyze it in the field as long as the stream is under enough positive pressure to drive the fluid through the sampling and analysis equipment, but when the stream is at low pressure (less than 5 psig) or under vacuum, there has historically been no way to take the sample and analyze it. The reason it is important to have contemporaneous chemical compositional analysis of a natural gas sample is because it affects both economics and safety, as explained more fully below.
In terms of economics, it is customary in the natural gas industry for producers of natural gas to be paid based on the BTU (British Thermal Unit) corrected volume of the natural gas stream. The BTU content of a natural gas stream is calculated based on chemical compositional analysis of a natural gas sample. For example, the relative amounts of nitrogen, oxygen, carbon dioxide, methane, ethane and propane components of a coal bed natural gas sample all affect its BTU content. Producers generally pay taxes and royalties based on the wellhead quantities of natural gas, and such taxes and royalties are based on the BTU corrected volume of the natural gas, also known as MMBTU (million BTUs). If the BTU content is over-estimated (for example, because it is not based on current data), the producer will overpay taxes and royalties. For this reason, it is highly desirable to ascertain the BTU content of the natural gas in the field so that the producer has real-time data for the natural gas that is produced at the wellhead.
As fields age, the concentration of non-combustible gases (nitrogen and carbon dioxide) in the natural gas generally increases, and toward the end of the production life of a coal bed natural gas well, the concentration of non-combustible gases in the natural gas emanating from the well usually spikes. As the concentration of non-combustible components increases, the concentration of hydrocarbon components (which typically include, but are not limited to, paraffins like methane, ethane and propane) decreases, thereby decreasing the BTU value. For each well, there is a point at which it no longer makes economic sense to produce from that well because of the increased non-combustible components and decreased hydrocarbon content of the natural gas. Having access to this information in the field allows operators to make timely decisions concerning the allocation of production resources.
Contemporaneous access to the results of chemical compositional analysis in the field is highly beneficial not only for economic reasons but also because it affects safety. Information about the chemical composition of the natural gas stream may let the operator know if there are leaks in the system that require immediate attention. For example, high levels of nitrogen in the sample might signify to the operator that there are leaks drawing air into the system. The presence of oxygen (presumably from the atmosphere) in the sample means there could be a risk of fire when the natural gas is compressed. If the operator becomes aware of increased nitrogen levels or the presence of oxygen in the system, he or she can take immediate steps to remedy the situation to prevent catastrophic loss or injury. Downstream pipelines that receive the produced natural gas may have compositional quality specifications that limit the amount of non-hydrocarbon gases (carbon dioxide, hydrogen sulfide, other sulfur species, and oxygen) allowed into their system to maintain the system integrity.
As noted above, mechanisms currently exist for taking natural gas samples and analyzing them in the field as long as the well is under enough positive pressure so that the natural natural will flow through the sampling and analysis equipment. Aside from the present invention, however, no mechanism exists for taking natural gas samples and analyzing them in the field when the well is at low pressure (less than 5 psig) or under vacuum. This creates problems for the well operators because they must extract a sample from the natural gas stream under vacuum conditions, deliver the sample to an offsite laboratory, and wait—usually several days and oftentimes up to two weeks—for the results of the chemical compositional analysis. This procedure lacks the immediate results that are obtainable when the chemical compositional analysis is conducted in the field. Furthermore, it is generally more cost-effective to analyze a sample in the field than to send it out to a laboratory since the lag time requires multiple visits to the well to input the updated chemical compositional analysis into the electronic flow computer, and if there was a problem with the initial sample, the process would have to be repeated until accurate data was obtained.
Accordingly, it is an object of the present invention to provide a natural gas sampling system that will allow samples to be taken and chemically analyzed to determine the composition of the fluid flowing from wells that are at low pressure (less than 5 psig) or under vacuum. It is a further object of the present invention to provide a means for such chemical compositional analysis to occur in the field. Yet another object of the present invention is to provide a low pressure and vacuum sampling system that is mobile and can be taken from field to field.